The present invention relates to a discharge processing device which can improve the processing efficiency while reducing the damage to switching elements at the time of discharge-processing a target.
The principle of target discharge processing and current waveforms required when the discharge processing is carried out will be explained first.
In a discharge processing, a discharge is started in the following manner: between a gap of an electrode and a target placed so as to face the electrode, after a minute conductive path not more than several tens gm has been found, a pulse current is allowed to flow through the minute conductive path so that the minute transport path or the electrode and a minute portion of the target contacting the path are forcefully evapotranspirated or fused and scattered by thermal energy generated at this point. In this case, the degree of evapotranspiration and fusing and scattering at the minute portion is determined by the following factors: the rate of change in time of the pulse current, that is, a current having an abrupt rising characteristic, the size of a current peak value, thermal characteristics of the electrode and target material and cooling characteristics of an insulating solution, etc.
When the target is made from a material having small electrical resistance, heat generation due to Joule heat becomes smaller. Furthermore, when the target is made from a material having superior thermal conductivity, the heat generation and temperature rise at the minute portion are lowered. Moreover, when the target is made from a material having high melting temperature, it is hardly melted even when heated. Furthermore, when the target is made from a material having high viscosity at the time of fusing, it is hardly scattered even when fused.
In an actual processing operation, these conditions are combined so that the resulting phenomena are a slow processing rate, a rough surface or a fine surface, susceptibility to short-circuiting, susceptibility to degradation in the processing efficiency and susceptibility to a concentrated discharge. Moreover, in the case of a wire discharge processing, the resulting phenomena are more short-circuiting and high frequency in wire disconnection.
Moreover, conventionally, in order to eliminate the short-circuiting phenomenon, alloys made of materials having a low fusing temperature and fusing latent heat or a low viscosity at the time of fusing, that is, materials having smoothness, have been used as the electrode material. Alloys such as brass are listed as these alloys; however, since these materials raise a problem of electrode consumption, etc., they are not used so much except for wire discharge processing machines and high-speed thin-pore processing machines. Here, a special wire which is coated with a material having a low fusing temperature and a small viscosity at the time of fusing has been developed as a wiring electrode for use in a wire discharge processing, and the application of such a special wire makes it possible to improve the processing efficiency while preventing the above-mentioned short-circuiting.
Moreover, after the start of the discharge, the insulating solution on the periphery of the discharge is evaporated so that a bubble that abruptly expands is formed. Thus, this internal pressure results in a reaction that scoops the fused portion out. As the discharging time elapses, the fused portion gradually expands, and as the bubble expands, the density of the generated inner pressure becomes smaller. Therefore, there is a greatest value in the amount of the scooped portion determined by the material and the discharging time, and both of a shortened discharging time (pulse width) and a lengthened discharging time cause degradation in the processing efficiency. In particular, even when the discharging time (pulse width) is lengthened exceeding the time required, the excessive discharging time is consumed as heat generation and fusing of the electrode and the target, resulting in a unwanted thick processed surface of the fused layer.
For this reason, respective conditions, which include the current peak value and the current rising rate that serve as discharge starting capabilities with the discharging time (pulse width) serving as a processing capability added thereto, are preferably set so as to be selected independently, depending on differences in thermal characteristics of the electrode, target and insulating solution. Moreover, when a pulse having a triangular waveform is used in such a very small area of a pulse width as described earlier, it is not possible to carry out an efficient processing operation.
When the same quantity of charge is applied in a rectangular waveform and a triangular waveform, the effective current value is smaller in the rectangular waveform. Therefore, when a shift is made from the triangular waveform to the rectangular waveform, heat input to the electrode can be reduced and the processing capability is improved. In particular, in the case of the wire discharge processing device, this arrangement is very effective since it prevents wire disconnection.
Japanese Laid-Open Patent Application No. 11-48039 discloses a discharge processing device in which the rising rate and pulse width of the above-mentioned electric current are independently controlled so as to provide an efficient processing operation. FIG. 30 shows a circuit structure thereof. Reference number 101 denotes a main dc power supply and reference number 102 denotes a sub dc power supply which supplies a voltage lower than the output voltage of the main dc power supply 101. Moreover, reference symbols T101, T102 and T103 denote first, second and third switching elements constituted by FETs.
The positive terminal of the main dc power supply 101 is connected to a target W through the first switching element T101, and the positive terminal of the sub dc power supply 102 is connected to the target W through the third switching element T103. Furthermore, the negative terminals of the main and sub dc power supplies 101 and 102 are connected to an electrode P through the second switching element T102. To the gates G101 to G103 of the FETs constituting the switching elements T101, T102 and T103 are connected switching element driving circuits (not shown), and the respective switching element driving circuits are allowed to on-off control the respective switching elements T101, T102 and T103 by using pulses output from a pulse distribution circuit (not shown).
FIG. 31 is a schematic drawing that exemplifies the relationship between the operational timing and the waveform of the discharging current (processing current) in the discharge processing device of FIG. 30.
When the discharge processing operation is started by the discharge processing device, pulse width setting data t1 and t2 are set in accordance with a dischargeable state between the electrode P and the target W. Based upon these settings, a pulse signal having the pulse width t2 is output from each of the switching element driving circuits, with the result that the second and third switching elements T102 and T103 are turned on as shown by (a) and (c) in FIG. 31.
As a result, the voltage of the sub dc power supply 102 is applied between the target W and the electrode P through the third switching element T103 and the second switching element T102 so that a current I1(xe2x95x90I0) flows from the sub dc power supply 102, thereby securing a current applying point (see(e) in FIG. 31). This is also referred to as a preliminary discharge which aims to secure the current applying point, and a separate power supply system may be installed for use in the preliminary discharge. The rise of this current is gradual since the output voltage of the sub dc power supply 102 is low. However, after a delay time successively set, a pulse having the time width t1 that has been set by the current peak value setting data is output from the rest of the switching element driving circuits, thereby turning the first switching element T101 on (see (a) in FIG. 31).
As a result, a current starts to flow from the main dc power supply 101 having a high voltage to form a processing current I0 that increases with an abrupt rise, as shown by (d) in FIG. 31, between the target W and the electrode P. Here, the period of this abrupt rise of the processing current I0 is defined as an initial processing period. When, after the lapse of the set time width t1, the first switching element T101 is turned off, the rise of the processing current I0 stops, and the current I0 is again supplied to the gap from the sub dc power supply 102 so that the processing current I0 is maintained at its peak value. Here, this period during which the processing current is maintained virtually at its peak value is defined as a mid process period.
After the lapse of the set time width t2 for use in setting the pulse width, the second and third switching elements T102 and T103 are turned off, and the currents I2, I3, which have been accumulated by an inductance within the circuit, are allowed to flow from the diode D101 to the target W to the electrode P to the diode D102 to the main dc power supply 101 so as to be fed back. At this time, since the feeding back is made to the main dc power supply 101 having a higher voltage, the falling rate of this processing current (I2=I3=I0) becomes abrupt.
Thereafter, this operation is repeatedly executed so that the discharge processing is carried out. Here, the processing current I0 after the stoppage of the application of the voltage is referred to as a last processing period.
As understood from FIG. 31, the processing pulse width is virtually determined by the pulse width setting data t2 that turns the second and third switching elements T102 and T103 on, and the peak value Ip of the processing current I0 is determined by the current peak value setting data t1 that turns the first switching element T101 on. Therefore, it is possible to easily set the current peak value Ip and the processing pulse width. Moreover, it is possible to provide an abrupt rise and an abrupt fall of the processing current I0 and consequently to maintain a processing current waveform having a virtually rectangular wave shape; thus, it becomes possible to improve the processing efficiency.
However, in the above-mentioned conventional discharge processing apparatus, the main dc power supply 101 applies a voltage between the electrode P and the target W so as to allow the discharging current to flow between them. Therefore, at the switching time of the initial processing period and the mid processing period, the current is supplied from the main dc power supply 101 through the switching element T101, and in a state where the current peak has been attained, the switching element T101 has to be turned off. The resulting problem is that the FETs constituting the switching element T101 have greater heat generation.
In order to reduce heat generation of the FETs constituting the switching element T101, high-performance heat-releasing fins have to be provided and multiple of these have to be aligned in parallel with each other. There is a problem in this that a switching element having a great capacity needs to be used as the switching element T101 and it becomes difficult to reduce costs.
Another problem arises in a current feed back in the last processing period. In other words, in the conventional method, after the stoppage of the voltage application from the sub dc power supply 102, the induced energy accumulated in the circuit is fed back to the main dc power supply 101 so that the current feed back is started from a voltage value that is determined by the main dc power supply 101. For this reason, the voltage value at the time of the current feed back tends to go beyond the power supply voltage, and this extremely high voltage has to be taken into consideration so that the voltage resistance of the switching element T101 has to be set high.
Therefore, the resulting problems are that the conventional technique requires high costs and is susceptible to malfunction.
It is an object of this invention to provide a discharge processing device which can start applying a processing current between the target and the electrode by using energy accumulated in a capacitor of the initial current supplying circuit so that it is possible to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current, to provide abrupt rising and falling portions of the discharge processing current waveform, and consequently to improve the discharge processing efficiency.
It is also an object of this invention to provide a discharge processing device which, even when the value of the voltage output from the capacitor of the initial current supplying circuit becomes lower than a voltage value preliminarily set, allows a diode to detect this fact so as to apply a current for use in mid processing period to flow between the target and the electrode, without the need of using a switching element driving circuit, etc. so as to on/off control each switching element so that it is possible to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current, to provide abrupt rising and falling portions of the discharge processing current waveform, and consequently to improve the discharge processing efficiency.
It is also an object of this invention to provide a discharge processing device which can prevent a charge from being accumulated in the capacitor of the initial current supplying circuit beyond a quantity required for the capacitor of the initial current supplying circuit so as to prevent disturbances in a processing current waveform of the next cycle and thereafter and consequently to provide an accurate control, so that it becomes possible to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current, to provide abrupt rising and falling portions of the discharge processing current waveform, and consequently to improve the discharge processing efficiency.
It is also an object of this invention to provide a discharge processing device which allows the capacitor of the initial current supplying circuit to accumulate a sufficient quantity of charge required so as to prevent disturbances in a processing current waveform of the next cycle and thereafter and consequently to provide an accurate control, so that it becomes possible to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current, to provide abrupt rising and falling portions of the discharge processing current waveform, and consequently to improve the discharge processing efficiency.
It is also an object of this invention to provide a discharge processing device which, even when the dc power supply contains floating inductance components, provides an abrupt rising portion in the processing current waveform so that it becomes possible to improve the discharge processing efficiency, to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, and consequently to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current.
It is also an object of this invention to provide a discharge processing device which, even in the case of a low arc voltage, can maintain a virtually rectangular processing current waveform so that it becomes possible to improve the discharge processing efficiency, to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, and consequently to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current.
It is also an object of this invention to provide a discharge processing device which prevents the current value of the mid processing period from becoming far greater than the current value of the initial processing period so as to maintain a virtually rectangular processing current waveform so that it becomes possible to improve the discharge processing efficiency, to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, and consequently to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current.
It is also an object of this invention to provide a discharge processing device which allows the capacitor of the initial current supplying circuit to maintain a sufficient charge so as to prevent disturbances in the processing current waveform in the next cycle and thereafter and consequently to provide an accurate control, so that it becomes possible to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current, to provide abrupt rising and falling portions of the discharge processing current waveform, and consequently to improve the discharge processing efficiency.
It is also an object of this invention to provide a discharge processing device which, prior to starting a processing operation on the target, allows the capacitor of the initial current supplying circuit to accumulate a sufficient charge so as to provide an abrupt rising portion of the discharge processing current waveform in the initial processing period, so that it becomes possible to improve the discharge processing efficiency, to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, and consequently to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current.
It is also an object of this invention to provide a discharge processing device which varies the layout of the capacitor of the initial current supplying circuit so as to further improve the degree of freedom in designing, so that it becomes possible to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current, to provide abrupt rising and falling portions of the discharge processing current waveform, and consequently to improve the discharge processing efficiency.
It is also an object of this invention is to provide a discharge processing device which, prior to starting a processing operation on the target, allows the capacitor of the initial current supplying circuit to accumulate a sufficient charge so as to provide abrupt rising and falling portions of the discharge processing current waveform in the initial processing period, so that it becomes possible to improve the discharge processing efficiency, to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, and consequently to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current.
It is also an object of this invention is to provide a discharge processing device which freely adjusts a charge to be accumulated in the capacitor of the initial current supplying circuit so as to provide an abrupt rising portion of the discharge processing current waveform in the initial processing period, so that it becomes possible to improve the discharge processing efficiency, to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, and consequently to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current.
It is also an object of this invention is to provide a discharge processing device which freely adjusts a charge to be accumulated in the capacitor of the initial current supplying circuit in a stepped manner so as to provide an abrupt rising portion of the discharge processing current waveform in the initial processing period, so that it becomes possible to improve the discharge processing efficiency, to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, and consequently to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current.
It is also an object of this invention to provide a discharge processing device which maintains a switching element having required voltage resistant and current capacity independent of the voltage resistance and current capacity of the switching element so as to reduce the costs of the entire device, so that it becomes possible to improve the discharge processing efficiency, while reducing the costs of the entire device, and consequently to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current.
The discharge processing device according to this invention processes a target by applying a processing current between the target and an electrode. This device comprises, an initial-stage current supplying circuit having a capacitor which supplies an initial current, which initial-stage current supplying circuit applies the processing current between the target and the electrode during an initial processing period; an intermediate-stage current supplying circuit having a dc power supply, which intermediate-stage current supplying circuit applies the processing current between the target and the electrode during an intermediate processing period; and a feedback circuit that charges the capacitor of the initial-stage current supplying circuit by dielectric energy accumulated during the intermediate processing period, during a last-processing period.
According to the above-mentioned aspect, in the discharge processing device which processes a target by applying a processing current between the target and the electrode, during the initial processing period, the initial-stage current supplying circuit having a capacitor of the initial current supplying circuit allows a processing current to flow between the target and the electrode. Furthermore, during the intermediate processing period, the intermediate-stage current supplying circuit having a dc power supply allows a processing current to flow between the target and the electrode. Finally, during the last-processing period, the feedback circuit converts dielectric energy accumulated during the intermediate processing period into an induced electromotive force so as to charge the capacitor of the initial current supplying circuit.
Accordingly, a processing current is allowed to start to flow between the target and the electrode by using the energy accumulated in the capacitor of the initial current supplying circuit. As a result, it is possible to use a switching element having a low voltage resistance and a low capacity so as to reduce the costs of the entire device, to simplify the adjustments of the peak value and the processing pulse width of the discharge processing current, to provide abrupt rising and falling portions of the discharge processing current waveform, and consequently to improve the discharge processing efficiency.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.